Turbofan case and method of making

THE FIELD OF THE INVENTION

0001This invention relates to gas turbine engines, and more particularly to a case for a turbofan engine.

BACKGROUND OF THE INVENTION

0002Affordable, “personal” jet aircraft are fast becoming a reality in the general aviation market, very small turbofan engines are required for power. Such aircraft require “very small” turbofan engines (i.e. 2000 pounds thrust and under) which can be economically operated by the general aviation pilot. Small scale turbofan gas turbine engines are known for use in expendable missiles in the delivery of military ordinance, however considerations such as cost-effective, affordable and efficient operation, and durability measured in thousands of hours (not minutes), have been irrelevant to their designers. Such prior art missile engine designs, therefore, provide none of the key deliverables required for such a market to be realized. Likewise, industrial microturbines are available, but their designs are ill-suited for use as an aircraft prime mover, for obvious considerations such as weight and size.

0003Scaling down of conventional civilian non-expendable turbofan engines, however, also presents difficulties due mainly to the disproportionate scaling of certain factors, such as strength to weight and tolerances. For example, non-expendable turbofan engines typically have a segmented case assembly, mainly for weight reduction reasons, but also to facilitate fabrication and assembly. A conventional case assembly <b>200</b> is illustrated in <figref idref="DRAWINGS">FIG. 1</figref>, and includes a fan case <b>202</b>, an intermediate case <b>204</b>, a compressor case <b>206</b>, a gas generator case <b>208</b>, a turbine case <b>210</b> and a turbine exhaust case <b>211</b> about centreline <b>212</b>. The gas generator case <b>208</b>, turbine case <b>210</b> and turbine exhaust case <b>211</b> surround the hot section of the engine and are typically made of steel or nickel alloys, which have good thermal resistance properties. However steel is relatively heavy, and therefore cooler portions such as the intermediate case <b>204</b> and the compressor case <b>206</b> typically employ lighter materials such as magnesium and/or aluminium. Steel is conventionally used for the fan case <b>202</b> because its strength is desirable for containing blade-off events.

0004A similar prior art configuration <b>300</b> is illustrated in <figref idref="DRAWINGS">FIG. 2</figref>, a case assembly <b>300</b> (only the upper half of which is shown), having a fan case <b>344</b>, an intermediate case <b>346</b>, and a gas generator case <b>352</b> (the turbine and exhaust cases are not shown) bolted together, along centreline <b>312</b>. A compressor shroud <b>348</b> for encircling the compressor blades is bolted to the intermediate case <b>346</b>, as is a bearing seat (not shown) at location <b>357</b>. Flange connections <b>302</b>, <b>304</b> and <b>306</b> are provided to accommodate differences in thermal expansion rates amongst the different material case components. Typically the case components are assembled in stages, as the engine component top-level assemblies are assembled therein.

0005Simply scaling down these larger case designs, however, becomes problematic in “very small” turbofan engines (i.e. generally 2000 pounds thrust, and less) for several reasons. One is the associated tolerance “stack-up”, which typically does not scale (i.e. the accuracy of manufacturing and assembly process does not increase as part size decreases). In typical turbofan engines, tolerance stack-up is less critical because it is small compared to the size of the components. But when considering blade tip clearance for example, the tolerance stack-up can have a very significant effect on the overall efficiency of a very small turbofan engine, since specific fuel consumption (SFC) is directly related to blade tip clearance. Any blade tip clearance must account for a tolerance stack-up, to avoid tip rubs caused by an unfavourable stack-up, and so tolerance stack-up directly affects efficiency. Another scaling problem is that factors often scale at different rates. For example, a component reduced to nominally half its original size, may not necessarily be halved in weight.

0006Another aspect which presents challenges to scaling down size is the differences in thermal expansion rates, which requires compensation and thereby adds weight and complexity. For example, the accessory gear box (AGB) tower shaft typically requires a telescoping design (and associated bearings) to account for thermal expansion differential. In the very small turbofan engine, such accommodations make the engine unfeasible expensive and inefficient to operate.

0007Therefore, as the affordable general aviation turbofan engine market develops, significant design problems are presented to the designer. Scaled-down turbofans are simply inefficient and heavy, and thus too expensive to operate in the general aviation market. Civilian version of expendable missile engines and airborne version of microturbines are also ineffectual solutions to the design problems presented. Thus, it is important to address the design problems of the very small turbofan engine.

SUMMARY OF THE INVENTION

0008One object of the present invention is to provide an improved gas turbine engine case for use in a very small gas turbine engine.

0009In accordance with one aspect of the present invention there is provided a casing for a turbofan engine which includes at least a fan assembly, a compressor assembly, a combustor assembly and a turbine assembly. The casing comprises a fan case portion, an intermediate case portion, and a gas generator case portion. The fan case portion, the intermediate case portion and the gas generator portion are integrally joined together, thereby forming an integral casing.

0010In accordance with another aspect of the present invention, there is provided a bypass turbofan engine. The bypass turbofan engine comprises at least a fan, a compressor, and a gas generator disposed in flow series within the engine, and a bypass airflow defined around at least the compressor and gas generator. A one-piece casing is provided, substantially encasing the fan, compressor and the gas generator.

0011In accordance with a further aspect of the present invention, there is provided a turbofan engine for aircraft, the turbofan engine comprising a rotating assembly which includes a propulsive fan portion, a compressor portion, and a gas generator portion. The rotating assembly has an axial length. A generally tubular casing assembly ias provided, enveloping the rotating assembly substantially along the axial length thereof, and thereby defining a main flow path through the engine. The casing assembly is an integrated single piece.

0012In accordance with a still further aspect of the present invention, there is provided a method of reducing the weight of a turbofan engine which includes a casing assembly. The method comprises a step of providing a one-piece integrated case to surround the turbofan engine and an associated bypass flow.

0013In accordance with a yet further aspect of the present invention, there is provided a method of assembling a gas turbofan engine for aircraft. The method comprises steps of providing a gas turbofan engine casing assembly including a fan case, an intermediate case and a gas generator case; placing a propulsive fan assembly, a compressor assembly, and a gas generator assembly into the casing assembly; and completing the assembly of the engine by mounting other components to the casing assembly.

0014It should be noted that the terms of “integral”, “integrating” and “integrated” used throughout the text of this application and appended claims, are intended to mean items which are integrally joined such that disassembly (in a typical non-destructive sense) is not possible.

0015Among other things, the integral turbofan engine casing of the present invention allows for a final machining operation to the casing assembly after assembly to reduce the tolerance accumulation in the assembly. Therefore, the present invention advantageously provides a method of assembling a turbofan engine in which a smaller minimum blade tip clearance and other stack-ups are achieved. The integral casing assembly also reduces the number of flange connections in the casing assembly which, despite the use of a typically heavier material throughout the casing, surprisingly reduces the overall weight of a very small turbofan engine. Furthermore, the integral engine casing also permits a much-needed reduction in thermal expansion differentials, thereby permitting a cost-efficient design to be provided for general aviation very small turbofan engines.

0016Other features and advantages of the present invention will be better understood with reference to the preferred embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

0017Having thus generally described the nature of the present invention, reference will now be made to the accompanying drawings, showing by way of illustration the preferred embodiments thereof, in which:

0018<figref idref="DRAWINGS">FIG. 1</figref> is a simplified exploded perspective view of a conventional case assembly of a turbofan engine;

0019<figref idref="DRAWINGS">FIG. 2</figref> is a schematic cross-sectional view of a similar conventional case assembly;

0020<figref idref="DRAWINGS">FIG. 3</figref> is a schematic cross-sectional view of a turbofan case according to the present invention;

0021<figref idref="DRAWINGS">FIG. 4</figref> is a schematic partial cross-sectional view of the embodiment of <figref idref="DRAWINGS">FIG. 3</figref>;

0022<figref idref="DRAWINGS">FIG. 5</figref> is a exploded isometric view, with a portion cut away, of an intermediate portion of the assembly of <figref idref="DRAWINGS">FIG. 4</figref>;

0023<figref idref="DRAWINGS">FIG. 6</figref> is an exploded isometric view of the assembly of <figref idref="DRAWINGS">FIG. 4</figref>, illustrating the assembly sequence of the intercase portion of <figref idref="DRAWINGS">FIG. 5</figref>;

0024<figref idref="DRAWINGS">FIG. 7</figref> is an isometric front view of the intercase portion shown in <figref idref="DRAWINGS">FIGS. 5 and 6</figref>;

0025<figref idref="DRAWINGS">FIG. 8</figref> is an isometric rear view of the intercase portion shown in <figref idref="DRAWINGS">FIGS. 5-7</figref>;

0026<figref idref="DRAWINGS">FIG. 9</figref> is an exploded and enlarged isometric front view of a portion of an alternate embodiment of the intercase portion of the present invention;

0027<figref idref="DRAWINGS">FIG. 10</figref> is an enlarged isometric front view of a cross-section of the assembled case of the present invention;

0028<figref idref="DRAWINGS">FIG. 11</figref> is an enlarged cross-sectional view of a portion of the present invention showing the fan exit vane installation; and

0029<figref idref="DRAWINGS">FIG. 12</figref> is a somewhat schematic cross-sectional view showing assembly steps according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0030Referring to the drawings, beginning with <figref idref="DRAWINGS">FIG. 3</figref>, an non-expendable exemplary turbofan gas turbine engine <b>10</b> according to the present invention includes in serial flow communication about a longitudinal central axis <b>12</b>, a fan assembly <b>13</b> having a plurality of circumferentially spaced fan blades <b>14</b>, a compressor section <b>16</b> having a plurality of circumferentially spaced low pressure compressor (LPC) blades <b>50</b> and high pressure compressor (HPC) blades <b>51</b>, a diffuser <b>18</b>, a combustor <b>20</b>, a high pressure turbine (HPT) <b>22</b>, and a low pressure turbine (LPT) <b>24</b>. LPT <b>24</b> is connected to the fan assembly <b>13</b> by a first or low pressure (LP) shaft <b>26</b>, and HPT <b>22</b> is connected to compressor assembly <b>16</b> by a second or high pressure (HP) shaft <b>28</b>. Fuel injecting means <b>30</b> are provided for injecting fuel into the combustor <b>20</b> assembly <b>16</b> by a second or high pressure (HP) shaft <b>28</b>. Fuel injecting means <b>30</b> are provided for injecting fuel into the combustor <b>20</b>.

0031A generally tubular casing assembly <b>32</b> having a envelops the engine <b>10</b> and thereby defines a main flow path <b>36</b> through the core of engine <b>10</b>, extending from an inlet <b>34</b> to an exhaust outlet (not shown), and a by-pass flow path <b>37</b>.

0032Referring to <figref idref="DRAWINGS">FIGS. 3</figref>, <b>4</b> and <b>6</b>, the casing assembly <b>32</b> according to one embodiment of the present invention includes a generally tubular fan portion or “case” <b>44</b>, which houses the fan rotor assembly <b>13</b>, a generally tubular intercase or intermediate portion or “case” <b>46</b> downstream of fan case <b>44</b> and a gas generator portion or “case” <b>52</b> downstream of intermediate portion <b>46</b>. The intermediate portion <b>46</b> includes a compressor shroud <b>48</b> which encircles the blade tips of the compressor assembly <b>16</b>, and a bearing seat <b>58</b> for mounting the HP shaft bearing <b>59</b> thereto, as will be described further below.

0033With reference to <figref idref="DRAWINGS">FIGS. 5 and 6</figref>, gas generator portion <b>52</b>, which is also generally tubular in shape, is for housing the combustor <b>20</b> and perhaps HPT <b>22</b> or a section thereof. A generally tubular case turbine and exhaust case <b>54</b> is preferably modularly provided and mounted to (i.e. not integrated with) the aft end <b>107</b> of gas generator case <b>52</b> for housing the LPT <b>24</b>, and supporting an exhaust mixer assembly (not shown).

0034The engine <b>10</b> further includes a tubular bypass duct case <b>56</b>, preferably modularly provided and mounted to (i.e. not integrated with) the intermediate portion <b>46</b> of casing assembly <b>32</b>. The tubular bypass duct case <b>56</b> generally surrounds the gas generator portion <b>52</b> and is radially spaced apart therefrom, thereby defining a downstream section of the bypass <b>44</b> therebetween.

0035Rather than providing a prior art segmented case, in which the case components are removably mounted to one another, the present invention provides a single-piece casing assembly <b>32</b> in which all casing components are integrally attached to one another. Referring again to <figref idref="DRAWINGS">FIG. 3</figref>, fan case portion <b>44</b>, intermediate case portion <b>46</b>, compressor shroud portion <b>48</b>, bearing mount <b>58</b> and gas generator portion <b>52</b> of casing assembly <b>32</b> are all integrally joined to one another, such as by welding, or by other process such as integral fabrication, brazing or other methods of joining and bonding the components into one piece. Preferably, the bypass duct case <b>56</b> is not integrated with casing <b>32</b>, in order to provide convenience in assembly and maintenance of the engine assembly <b>10</b>, and so rather is connected by bolting together mating flanges <b>60</b> and <b>62</b> which extend radially from the respective intermediate portion <b>46</b> and the bypass duct case <b>56</b>. The turbine and exhaust case <b>54</b>, as mentioned, is also preferably mounted to the aft end of the casing <b>32</b> by, for example, bolting together mated flanges <b>64</b> and <b>66</b>. The bypass duct <b>56</b> and the case <b>54</b> are shown by broken lines in <figref idref="DRAWINGS">FIG. 4</figref> to distinguish them from other cases which are most preferably integrated to form the integral case of the present invention. Casing assembly <b>32</b> can also integrally include the bypass and exhaust ducts, if desired.

0036The individual components of casing <b>32</b> are preferably made from one material, for example steel, although a combination of materials may be used (e.g. steel and Inconel, etc.) as long as the desired integral bonding technique (e.g. welding) permits such materials to be reliably bonded together. The individual portions of the casing are preferably made separately, as will be described further below, which would permit, for example, a variety of processes and materials to be used. Optionally, the casing <b>32</b> may be formed integrally substantially in a single operation, such as metal injection moulding.

0037Surprisingly, although the entire casing <b>32</b> of the present invention may be made from a relatively heavy material such as steel, in very small turbofan engines (i.e. preferably 2000 pounds thrust and less, more preferably 1500 pounds thrust and less, and most preferably about 1000 pounds thrust or less) the present invention provides unexpected and significant benefits which directly impact on engine SFC, as will now be described.

0038Firstly, even though a heavier material is used throughout (e.g. steel versus, say, magnesium), the weight savings from reduced flange count is surprisingly significant. Even scaled-down flanges represent a significant weight relative to the very small turbofan engine, and thus it has been found that their removal results in a disproportionate weight savings despite the addition of weight elsewhere in the casing, contrary to the teachings of the prior art. Therefore, contrary to the teachings of the prior art, it has been found that a segmented case permitting the use of lighter materials is actually heavier in the very small turbofan range. A beneficial redistribution of weight is therefore provided by the present invention.

0039Secondly, the reduction of flange connections also beneficially reduces tolerance stack-up by reducing the number of toleranced parts and connections. Accordingly, for example by integrating the compressor bearing mount and compressor shroud into a single part, a significantly smaller compressor blade tip clearance may be provided.

0040Thirdly, the reduction of thermally mismatched parts also permits a significant simplification to the very small turbofan engine. In a first aspect, the reduction of thermal mismatch improves the tolerances which must be left in connections. In a second aspect, by improving thermal mismatch within the casing <b>32</b>, the interface with other systems, such as the accessory gearbox (AGB) is greatly simplified.

0041In a second aspect of the present invention, a configuration for casing <b>32</b> is disclosed which provides further benefits to the very small turbofan. Referring to <figref idref="DRAWINGS">FIGS. 4 and 5</figref>, the structure of the intermediate portion <b>46</b> of casing <b>32</b> will now be described in more detail. The intermediate portion <b>46</b> includes an annular outer portion or outer ring <b>68</b> having a forward end <b>70</b> and a rearward end <b>71</b> integrated with the radially outwardly extending bypass duct flange <b>60</b>. On the external surface of the outer ring <b>68</b> are provided stabilizing ribs <b>72</b>, which reinforce the rigidity of the outer ring <b>68</b>, and engine mounts <b>74</b> which also assist in this regard. A mounting support <b>82</b> on the outer ring <b>68</b> is provided for operatively supporting the AGB tower shaft (not shown), and to provide further stiffness to ring <b>68</b>. Also provided on the outer ring <b>68</b> are attachment brackets <b>84</b> for attaching the AGB. Other services, such as oil tube inlet <b>83</b> and Ni probe boss <b>85</b>A, are also provided.

0042The intermediate portion <b>46</b> of casing <b>32</b> also includes an annular inner portion including an inner hub <b>76</b> which has a forward end <b>78</b> and a rearward end <b>80</b>. The inner hub <b>76</b> is positioned coaxially with the outer ring <b>68</b> and is supported within the outer ring <b>68</b> by a plurality of casing struts <b>40</b> which are circumferentially spaced apart and extend radially outwardly and generally rearwardly from the inner hub <b>76</b> to the outer ring <b>68</b>, as will be described further below. The annular bearing seat <b>58</b> which receives and supports preferably the HPC bearing <b>59</b> (see <figref idref="DRAWINGS">FIG. 3</figref>) is integrally attached (for example, by welding, as described below) to the rearward end <b>80</b> of the inner hub <b>76</b>. A mounting flange <b>77</b> is also provided on the forward end <b>78</b> of the inner hub <b>76</b> (see <figref idref="DRAWINGS">FIGS. 4 and 5</figref>) for attaching a forward bearing housing (not shown) for the LP shaft bearings.

0043The annular inner portion of the intermediate portion <b>46</b> of casing <b>32</b> also includes a splitter <b>42</b>, which includes an annular inner wall <b>85</b> and an annular outer wall <b>86</b> extending axially and downstream relative to the air flow through engine <b>10</b>, divergent from an annular leading edge tip <b>88</b>. A section of the annular bypass path <b>37</b> is thereby defined between the outer ring <b>68</b> and the annular outer wall <b>86</b> of the splitter <b>42</b>, while core flow path <b>36</b> is defined between the annular inner wall <b>85</b> of the splitter <b>42</b> and the inner hub <b>76</b>. A stiffener <b>94</b> is provided within splitter <b>42</b>, between the inner and outer walls <b>85</b>, <b>86</b>, and affixed thereto, and preferably also affixed to struts <b>40</b>, as will be described further below. As described previously, the compressor shroud <b>48</b>, which is preferably thicker than the inner wall <b>85</b> of the splitter <b>42</b> to withstand the demands of the compressed air flow, is integrated (for example by welding, as described further below) to the inner wall <b>85</b>.

0044A plurality of circumferentially spaced apart slots <b>90</b> extend generally from near the annular tip <b>88</b> axially into the splitter <b>42</b>, for receiving the respective casing struts <b>40</b>. A plurality of corresponding bosses <b>91</b> and <b>93</b> are respectively provided in the inner hub <b>76</b> and the outer ring <b>68</b> for attaching the casing struts <b>40</b>.

0045A bleed valve housing <b>92</b> (see <figref idref="DRAWINGS">FIGS. 4 and 6</figref>) is preferably attached by welding, to the annular outer wall <b>86</b> of the splitter <b>42</b> at its rearward end, for securing bleed valve(s) (not shown) thereto. The intermediate portion <b>46</b> also bleed holes <b>96</b> defined in the outer wall <b>86</b> of the splitter <b>42</b>, for co-operation with an air bleed system (not shown). Bleed holes <b>96</b> are preferably made when fabricating the splitter <b>42</b>.

0046Though when assembled it has the appearance of a prior art intercase, which is most typically cast, the present invention advantageously permits the individual components of intermediate portion <b>46</b> may be made in accordance with a variety of manufacturing processes. The preferred processes will now be described. Outer ring <b>68</b> and inner hub <b>76</b> are machined from solid. Outer ring is generally quite thin (i.e. sheet-metal-like) and, in conjunction with stiffeners <b>72</b>, provide intercase portion <b>46</b> with a semi-monocoque construction which is lightweight yet strong. Service attachments, such oil tube inlet <b>83</b> and N<b>1</b> probe boss <b>85</b>, are cast (or metal injection moulded, forged, machined, etc., as desired) and welded or brazed to outer ring, while other “attachments” such as tower shaft support <b>82</b> are integrally machined with the ring. Struts <b>40</b> are formed preferably in sheet metal halves (though processes such as metal injection moulding, hydroforming, flow forming, casting, etc. may be used) and then integrally joined by welding to provide a hollow configuration. One strut preferably receives an AGB tower shaft (not shown), another the oil tube and N<b>1</b> probe (not shown), and so on. The struts <b>40</b> are preferably welded to bosses <b>91</b> and <b>93</b> and within slots <b>90</b>, to thereby assemble outer ring <b>68</b>, splitter <b>42</b> and inner hub <b>70</b> to provide intercase portion <b>46</b> of casing <b>32</b>.

0047Referring to <figref idref="DRAWINGS">FIG. 9</figref>, in an alternate embodiment, intercase portion <b>46</b> may have struts <b>40</b> which have a configuration which provides a modified joint with splitter <b>42</b> and outer ring <b>68</b>, through the inclusion of flanged components <b>40</b>A and <b>68</b>A which may be welded to struts <b>40</b> and outer ring <b>68</b> respectively. Such flanged components may be provided to facilitate stronger connection welds, etc. and thus this embodiments further illustrates the flexibility the present invention gives the designer.

0048The individual components are integrated together preferably by welding (or other integral joining technique of the general types already mentioned) to provide the integrated intermediate portion <b>46</b>, and this is preferably before integrating the intermediate portion <b>46</b> with the other portions of the casing <b>32</b> (i.e. fan portion <b>44</b>, etc.). The details of the intermediate portion <b>46</b> may vary depending on various embodiments used for various engine models.

0049Referring to <figref idref="DRAWINGS">FIGS. 4 and 6</figref>, the fan portion <b>44</b> includes an annular upstream section <b>98</b> encircling the fan blades <b>14</b> (see <figref idref="DRAWINGS">FIG. 3</figref>). The upstream section <b>98</b> is preferably strong enough to ensure containment of a blade-off incident, or incorporate an insert therefor (not shown). The fan case <b>44</b> includes a downstream section <b>100</b> which extends from the upstream section <b>98</b> to a downstream edge <b>103</b>. The downstream section <b>100</b> incorporates slots <b>101</b> which locates and supports the outer end of fan exit vanes <b>38</b>, as will be described below.

0050Referring to <figref idref="DRAWINGS">FIG. 10</figref>, the stator-less fan exit vanes <b>38</b> are slidingly inserted preferably from outside the fan portion <b>44</b> and therefore slots <b>101</b> are defined accordingly in the section <b>100</b> of the fan portion <b>44</b> (see <figref idref="DRAWINGS">FIG. 6</figref>) and in the inner shroud <b>102</b>. The fan exit vanes <b>38</b> are releasably mounted between the section <b>100</b> of the fan portion <b>44</b> and the inner shroud <b>102</b> at the corresponding slots, and releasably retained therein by pliable compression-fit insert grommets <b>120</b> (see <figref idref="DRAWINGS">FIG. 11</figref>) and straps <b>122</b>.

0051Fan portion <b>44</b> may be flow-formed from one material, such as steel, nickel or inconel. Alternate fabrication or forming techniques may also be used, and one or more materials may be used.

0052The fan portion <b>44</b> is integrated into the intermediate portion <b>46</b> by integrally joining, preferably by welding, the aft end <b>103</b> of fan case portion <b>44</b> with the forward end <b>70</b> of the outer ring <b>68</b> of the intermediate portion <b>4</b> to thereby create an integral joint <b>130</b> (see <figref idref="DRAWINGS">FIG. 4</figref>). The inner shroud <b>102</b> of the fan portion <b>44</b> is also attached to the inner hub <b>76</b> of the intermediate portion <b>46</b>, preferably by welding at <b>132</b>. The inner shroud <b>102</b> and the fan exit vanes <b>38</b> are preferably not integrated with the casing assembly <b>32</b>, but rather are releasably mounted to the fan portion <b>44</b> as described above after the fan portion <b>44</b> is integrated with the intermediate portion <b>46</b>.

0053The gas generator case portion <b>52</b> of casing <b>32</b>, includes a upstream section <b>104</b> and a substantially cylindrical downstream section <b>106</b> which are integrated together, preferably by being fabricated in a single manufacturing process. An integral inner ring <b>108</b> is disposed within the upstream section <b>104</b> and is integrated, preferably by welding, with the gas generator case <b>52</b> at the forward end thereof. A mounting flange <b>110</b> extends radially outwardly from the inner ring <b>108</b> at the inner edge thereof, for securing the diffuser <b>18</b> flange <b>110</b>A and bleed valve <b>150</b> thereto (see (<figref idref="DRAWINGS">FIGS. 3</figref>, <b>4</b> and <b>12</b>). A number of openings <b>140</b> (see <figref idref="DRAWINGS">FIG. 6</figref>) are provided in the gas generator case <b>52</b> for receiving or mounting engine components of the gas generator portion, such as fuel injecting means <b>30</b>, and so on, as will be understood by one skilled in the art. The downstream cylindrical section <b>106</b> has an aft end <b>107</b> which is integrated with a radially outwardly extending mounting flange <b>112</b>, for connection with turbine and/or exhaust case <b>54</b>. The gas generator case <b>52</b> is integrated at the front end thereof with the aft end <b>89</b> of the annular outer wall <b>86</b> splitter <b>42</b> of the intermediate portion <b>46</b> at <b>134</b>, also preferably by welding.

0054The fan portion <b>44</b>, the intermediate portion <b>46</b> and the gas generator portion <b>52</b> of casing <b>32</b> are thus fabricated separately, for example by machining from solid, sheet metal fabrication, forging, casting, flow-forming, etc., depending on the design of each and the wishes of the designer. The separately fabricated cases are then integrally attached preferably by welding. It is then preferable to finally machine the interior portions of the integrated casing <b>32</b> prior to installation of rotor assemblies, in order to reduce any tolerance stack-up occurring during casing <b>32</b> manufacture or assembly. This dramatically reduces the tolerance stack-up over prior art devices.

0055The way in which each portion is formed and the exact means by which the the portions are attached are not critical to the invention, but rather may be left to the designer's discretion. Therefore, the present invention allows for flexibility in selection of manufacturing processes to meet the designer's needs in providing an integrated case assembly for a very small turbofan engine. The present invention thereby permits a variety of manufacturing techniques, notably among them fabrication techniques such as machining from solid, flow-forming and sheet metal construction, which are not available with prior art casing designs.

0056In yet another aspect of the present invention, the flexibility of manufacture permitted by the present invention permits the bearing mounts integrally provided in the case to be much simpler, in terms of part count, than prior art bearing mounts. Typical prior art gas turbine engines require complicated bearing mounts, including assemblies known as “squirrel cages” to dampen vibrations caused by rotor imbalances which inevitably result despite highly accurate machining processes. In the present invention however, bearing mounts such as bearing mount <b>58</b> may be provided with an integrated flexibility, such that which is a function of its material, configuration, stiffness, etc., such that bearing mount <b>58</b> itself can be “tuned” during manufacture to thereby obviate the need for a squirrel cage. The bearing mount <b>58</b> is thus integrally designed and provided to also perform a damping function to remove the need for separate squirrel cage assemblies. Since squirrel cages add weight, length and complexity to the engine, deleting this component is of course valuable and therefore yet another beneficial feature of the present invention.

0057Referring now to <figref idref="DRAWINGS">FIGS. 5</figref>, <b>6</b> and <b>12</b>, in a yet further aspect of the present invention, a method for assembling a turbofan engine will now be described. Unlike the prior art, the present invention casing <b>32</b> is preferably fully (or substantially) assembled before any rotating or other gas turbine components are assembled therein. Thus, the first step is making and assembling the components of the casing assembly <b>32</b>, as described above. The next step, also described above, preferably is to machine internal surfaces of the casing <b>32</b>, such as surfaces relating to bearing mounts, compressor shrouds and similar surfaces, to remove any accumulated tolerance stack-up which would affect the efficient operation of the engine. The next steps are to insert the fan rotor assembly <b>13</b> inside casing <b>32</b> (step not shown in the Figures), preferably through the inlet <b>34</b> of the casing assembly <b>32</b> and into the fan portion <b>44</b>, and to insert the bleed valve <b>150</b> and compressor assembly <b>16</b> into casing <b>32</b>, preferably through gas generator portion <b>52</b> (see <figref idref="DRAWINGS">FIG. 12</figref>). The diffuser <b>18</b>, combustor <b>20</b>, the turbine assemblies, and other components are also inserted into casing <b>32</b>, also preferably from the aft end of the gas generator portion <b>52</b>. The assembly process of the engine <b>10</b> is then completed by further mounting the turbine and exhaust case <b>54</b>, the bypass duct <b>56</b>, and other engine components in and to the casing assembly <b>32</b>. While the specific order of insertion and assembly of these interior assemblies in casing may depend on preference or the design layout of engine <b>10</b>, the present invention involves building the core of engine <b>10</b> inside a completed or substantially completed casing <b>32</b>, thereby permitting an overall more efficient assembly technique for the gas turbine engine.

0058The present method also advantageously provides a fast assembly of a gas turbine engine because no fixtures such as flange connections are required and therefore, less “final” assembly steps are required.

0059As mentioned, the present invention has particular application for use in so-called very small gas turbine engines, namely engines typically 2000 pounds thrust and below for use in general aviation aircraft sometimes referred to as “personal” jet aircraft. This market represents a leading edge of gas turbine turbofan technology, wherein the limits of scaling and cost-effective design and operation are challenged. Prior art small turbines, such as those used in missile engines are simply unsuitable. Missile engines are invariably expensive to make and operate (owing to their military heritage), and are designed for extremely short operational lives (a few hours) in which they are continuously operated at full thrust. The very small turbofan as contemplated herein, however, must of course be operated intermittently at varying thrust levels (e.g. idle, taxi, take-off, climb, cruise, approach and landing) for thousands of hours, not to mention be affordable and quiet to operate and environmentally friendly. Likewise, although microturbines are beginning to proliferate in the power generation field, this technology is also largely unsuitable since aircraft applications require extremely lightweight and reliable designs which are typically not found in industrial microturbine designs. Accordingly, the present invention represents an advance in the field of providing an affordable-to-operate turbofan to general aviation pilots.

0060The present invention permits a turbofan casing to be provided which, in the very small turbofan size range, permits the overall weight of the casing to be reduced over conventional larger designs. The weight reduction is due in part to the thin shell stiffened semi-monocoque design of the intermediate case section, which has an integrally-stiffened thin shell construction which allows the designer to optimize the use of metal to thereby reduce weight. The thin “sheet” outer ring “panels” are reinforced at specific locations by the ribs and struts, and by engine mounts and other similar features on the ring, to balance external loading by compression and tension in the reinforcing members reacting balanced shear in the “panels” of the outer ring. This provides a stable structure with a stiffness comparable to a cast structure more than 500% thicker. It is through this approach, combined with the simplicity of attachment, that the overall weight of the casing is significantly reduced.

0061While the above description addresses the preferred embodiments, it will be appreciated that the present invention is susceptible to modification and change without departing from the scope of the accompanying claims. For example, while described in respect of an application to very small turbofan engines, some benefits may be attained in larger turbofan or other gas turbine engines in applying the principles of the present invention. Though the use of certain materials and manufacturing methods have been disclosed as preferred, other materials and methods may be substituted without departing from the present invention. The turbine and exhaust cases may also be integrated if desired into a casing according to the present invention, either together with, or in place of some of the components described above. For example, an integrated fan and intermediate case could be attached in a conventional fashion to an integrated gas generator and turbine case. The compressor shroud integrated with the casing could be an LP or HP compressor shroud. Still other modifications will be apparent to those skilled in the art which will fall within the scope of the invention intended by the inventors, and the appended claims should be interpreted to incorporate such modifications.